Gimbal systems that offer precision laser marker/designator capability and that are relatively small (e.g., gimbal diameter of less than about 5″) do not currently exist. In the context of the present invention, a gimbal system refers to a gimbal assembly (i.e., a gimbal) and system components (e.g., payload assets and associated electronics) mounted thereon. Currently available gimbal systems that are suitably small for use in precision applications such as, for example, relatively small unmanned air vehicles (UAVs) do not meet pointing performance requirements needed for target designation. Some of these UAV platforms require pointing accuracies measured in a few degrees or less. In such UAV applications, a suitably high resolution of pointing accuracy is required as it is necessary to accurately hold a laser designator spot on a target for delivering laser-guided ordinance. As an example, UAV applications can require a laser beam stabilization of less than about 25 μrad.
A two-axis azimuth/inner elevation (i.e., el-over-az) gimbal is a well-known configuration of gimbal and is preferred configuration of gimbal for many applications. One reason a two-axis el-over-ax gimbal system is preferred for certain applications is because its low number of pivot axes corresponds to an overall construction that can offer relatively low weight. However, its low number of pivot axes has an adverse impact on its pointing performance.
A two-axis el-over-az gimbal consists of three main subassemblies. A base subassembly (i.e., a gimbal base structure of the gimbal) and provides mounting features and interconnect to a vehicle or other form of apparatus on which it is installed and, thus, the base is fixed with respect to the vehicle. An azimuth subassembly (i.e., a gimbal azimuth structure of the gimbal) rotates about the base subassembly, typically along an axis normal to the primary mounting plane of the base. An elevation subassembly (i.e., a gimbal elevation structure of the gimbal) is supported by the azimuth subassembly and rotates about an axis normal to the axis of rotation of the azimuth subassembly. Each one of the pivoting gimbal subassemblies is a nested axis subassembly of a gimbal.
The axis of rotation of the elevation subassembly typically contains a payload comprising a suite of sensors and/or indicators (i.e., payload assets), such as for example cameras, laser range finders (LRF), and laser pointers/markers/designators. The payload assets are typically pointed such that lines of sight of the sensors and indicators are in a direction normal to the axis of rotation of the elevation subassembly. This allows the payload assets to be directed in an arbitrary direction by moving the azimuth and elevation subassemblies.
It is well known that high power lasers are used to identify targets to other systems such as, for example, laser-guided munition systems. Along with their inherent size, packaging a targeting laser requires considerations for thermal stabilization and dissipation. As such, the subassembly that carries the laser must have sufficient characteristics for meeting thermal stabilization and dissipation requirements as well as overall space requirements. Furthermore, in order to have the laser output follow the pointing direction of the other payload assets (i.e., be boresighted to the other payload assets), the output of the laser (i.e., laser beam) must pass into the elevation subassembly before being transmitted out of the gimbal system. One approach for this is referred to a Coudé path in which the laser beam passes from the azimuth subassembly to the elevation subassembly along the elevation subassembly rotational axis. Through such a path arrangement, the laser beam incoming to the elevation subassembly is in generally the same location regardless of the angle of rotation of the elevation subassembly.
In order to maintain an acceptable spot size on the target, the divergence of the laser beam must be low. This is typically achieved using a beam expanding telescope, which reduces divergence while increasing beam diameter, as the divergence at the output of the laser is unacceptably high. For precision targeting applications, the location of the spot generated by the laser beam on the target must not be influenced by motion of the vehicle by variables such as, for example, aerodynamic disturbance. Unintended spot movement is commonly referred to as jitter in the context of high frequency errors and unintended spot movement is commonly referred to as pointing error in the context of low frequency or constant errors. A gimbal system itself is designed to reject disturbances and provide a stabilized platform for the payload assets.
Stabilization requirements for a laser, particularly those used for pointing/designating purposes, exceed the stabilization capability achievable by relatively small two-axis gimbal systems (e.g., those with a gimbal diameter of less than about 5″). This problem is sometimes overcome in larger gimbal systems by using additional nested axis subassemblies. These additional nested axis subassemblies can null-out disturbances not rejected by the subassemblies of outer axes. However, high-count nested axis gimbal system configurations are undesirable and/or unsuitable in many applications (e.g., small UAV applications) because they are too heavy and large for such applications. Therefore, a 2-axis gimbal system that is relatively small and that offers stabilization of gimbal systems with additional nested axis subassemblies would be advantageous, desirable and useful.